1. Field of the Invention
The present invention relates to a developing device for developing a latent image formed on an image carrier with a magnet brush formed on a developer carrier and a copier, printer, facsimile apparatus or similar image forming apparatus including the same.
2. Description of the Background Art
Higher image quality and higher durability both are required of a modern image forming apparatus. More specifically, image quality should be little susceptible to varying environmental conditions and stable despite aging. It is a common practice with a developing device included in an image forming apparatus to use either one of a single-ingredient type developer, i.e., toner only and a two-ingredient type developer that is a mixture of nonmagnetic toner and magnetic carrier. Today, the two-ingredient type developer is predominant over the one-ingredient type developer because of various merits particular thereto. However, a developing device using the two-ingredient type developer has the following problems left unsolved.
A first problem is the omission of the trailing edge of an image. Generally, this problem occurs more frequency as the ratio of the linear velocity Vs of a developing sleeve to the linear velocity VP of a photoconductive element (Vs/Vp) increases. More specifically, this kind of omission refers to an occurrence that the trailing edge of a halftone portion positioned at the downstream side in the direction of sheet feed is short of density or is not developed at all. A second problem is that thin lines cannot be faithfully reproduced, i.e., the ratio of the width of vertical lines to that of horizontal lines increases to 1.4 or above.
Japanese Patent Laid-Open Publication No. 7-140730, for example, discloses an image forming apparatus configured to solve the problems described above. The image forming apparatus disclosed includes a magnet brush type developing unit including a developer carrier and a magnet roller fixed in place in the developer carrier and having a plurality of magnets. The main pole of the magnet roller for development is positioned at an angle of 5xc2x0 to 20xc2x0 upstream of a plane containing the center of the magnet roller and that of the image carrier in the direction of developer conveyance. A doctor member also included in the developing unit and the developer carrier are spaced from each other by a distance Hcut ranging from 0.25 mm to 0.75 mm. A nip for development extends over a distance Dsd of 0.3 mm to 0.8 mm. The distances Hcut and Dsd are selected to satisfy a relation of 1.20 less than Dsd/Hcut less than 1.60. Further, the linear velocity Vs of the developer carrier and the linear velocity Vp of the image carrier are selected to satisfy a relation of 1.0xe2x89xa6Vs/Vpxe2x89xa63.0. The above document describes that such a configuration obviates sweep marks, i.e., disturbances to a toner layer in a halftone portion and a solid portion to thereby produce images with high, uniform density and clear-cut contours at high speed.
Further, the above Laid-Open Publication No. 7-140730 teaches a plurality of developing units each including a respective sleeve having a plurality of magnetic poles. The developing positions on the sleeves are different from each other color by color; a latent image is developed between magnetic poles by a non-contact system. Two poles sandwiching a developing zone have an intensity of 500 gausses or above each and are spaced from each other by an angle xcex8 above the range of 40xc2x0 to 70xc2x0. Further, a magnet angle xcex81 is selected to be between 0xc2x0 and one-tenth of the above angle xcex8 or less. The document describes that such a configuration stably produces high-quality images with a minimum of fog ascribable to the carrier deposited on the image carrier or a minimum of local omission around a portion where the carrier is deposited.
The omission of portions around a character is a problem recently reported in relation to the developing device using the two-ingredient type developer in addition to the omission of trailing edges. The omission of portions around a character also refers to an occurrence that dots forming a halftone portion are short of density or are not developed at all. However, this kind of omission differs in position from the omission of a trailing edge. More specifically, the omission of a trailing edge occurs when a halftone patch adjoins a non-image portion, the trailing edge of halftone is lost. As for the omission of portions around a character, when a character portion exists in a halftone region (1xc3x971 dot of, e.g., 26% dot), i.e., when the trailing edge of halftone adjoins a solid image region (character region), the halftone portion of the character region is lost.
In the developing device taught in the above Laid-Open Publication No. 7-140730, the distances Hcut and Dsd satisfy a relation of 1.20 less than Dsd/Hcut less than 1.60, as stated earlier. This, however, makes the magnet brush around the point where the sleeve and photoconductive element are closest to each other more rough as the ratio Dsd/Hcut noticeably varies from 1, i.e., as Hcut decreases relative to Dsd. It is true that such a condition enhances the faithful reproduction of horizontal lines and reduces the omission of trailing edges. However, the magnet brush cannot uniformly contact or rub the entire surface of the photoconductive element, resulting in the omission of portions around characters. Moreover, as for a halftone image with density lying in the range of 0.3 to 0.8 (ID), the magnet brush failing to uniformly contact the photoconductive element cannot uniformly reproduce a dot image, causing the halftone image appear granular.
Japanese Laid-Open Publication No. 6-149063 proposes a non-contact type developing device using the two-ingredient type developer. Non-contact type development, however, lacks an intense electric field for development and cannot be easily improved in developing ability. As a result, this type of developing device aggravates the omission of portions around characters although improving the omission of trailing edges and the faithful reproduction of thin lines.
As stated above, it is difficult with the conventional developing devices using the two-ingredient type developer to improve all of the thinning of horizontal lines, the omission of trailing edges and the omission of portions around characters at the same time.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2001-27849.
It is an object of the present invention to provide a developing device capable of reducing the omission of portions around characters while obviating the thinning of horizontal lines and the omission of trailing edges to thereby reduce defective images, and an image forming apparatus including the same.
A developing device of the present invention includes a developer carrier facing an image carrier, which carries a latent image thereon, via a gap to thereby form a developing zone between the developer carrier and the image carrier. The developer carrier includes a main magnetic pole for development and magnetic poles respectively positioned upstream and downstream of the main magnetic pole in a direction of developer conveyance for conveying a developer. The developer forms a magnet brush on the surface of the developer carrier. The main magnetic pole has an angle of 60xc2x0 or below between its opposite pole transition points respectively positioned upstream and downstream of the main pole in the direction of developer conveyance. A flux density between said main magnetic pole and the magnetic pole downstream of the main magnetic pole in the normal direction has a peak value that is 80% of the maximum flux density of the main pole in the normal direction or above.
An image forming apparatus including the above developing device is also disclosed.